The invention is intended especially for use in validating coins having an inner, central core made of a first metallic material and an outer ring made of a second metallic material. Such coins are commonly known as bi-colour coins. The invention is also useful for coins having two or more outer rings of different compositions. One or more of the core and outer ring(s) may be formed of layers of two or more materials, in a "clad" construction.
The term coin is used throughout the specification to mean any coin (whether genuine or counterfeit), token, slug, washer, or other metallic object or item, and especially any metallic object or item which could be used in an attempt to operate a coin-operated device or system. A "valid coin" is considered to be an authentic coin, token, or the like, of an acceptable denomination and which a coin-operated device or system is intended selectively to receive and to treat as an item of value, and especially an authentic coin of a monetary system or systems in which or with which a coin-operated device or system is intended to operate.
Various techniques for validating coins and, in particular, for testing the material of coins, are known. Coin testing apparatus is well known in which a coin is subjected to a test by passing it through a passageway in which it enters an oscillating magnetic field produced by an induct-or and measuring the degree of interaction between the coin and the field, the resulting measurement being dependent upon one or more characteristics of the coin and being compared with a reference value, or each of a set of reference values, corresponding to the measurement obtained from one or more denominations of acceptable coins. It is most is usual to apply more than one such test, the respective tests being responsive to respective different coin characteristics, and to judge the tested coin acceptable only if all the test results are appropriate to a single, acceptable, denomination of coin. An example of such apparatus is described in GB-A-2 093 620.
More specifically, it is known from EP 0 710 933 to test bi-colour coins using an inductive sensor, in the form of pair of coils, in combination with two optical sensors. in the apparatus described in EP 0 710 933 the optical sensors are used to control the operation of the inductive sensor to produce a first reading of the coin when the coin is centred on the coils and a second reading when the outer rim portion of the coin is centred on the coils, that is, when the rim portion in combination with other adjacent portions of the coin are in the field of the sensors.
A disadvantage of the device mentioned above is that, if an optical sensor becomes dirty, the accuracy of the timing of the reading of the inductive sensors, which is controlled by the optical sensors, may be reduced. Further, the optical sensor may fail to operate altogether if, for example, the light source or detector is blocked by a piece of dust. Another disadvantage is that the device uses a measurement taken when both the outer rim material and the centre material of the coin, and thus the interface between the two materials, are within the field of the coils for validating the coin. It has been found that the effect on an inductive sensor of a portion of a bi-colour coin including the interface between the two materials changes over the life of a coin, and also it will not necessarily be the same for all coins of the same type, so that coin validation based on a measurement taken over the interface may not be accurate. All the above disadvantages can lead to a valid coin being rejected or an invalid coin accepted.